The invention relates to a flexographic printing machine, especially for flexographic web printing, which comprises at least one printing unit, each with an endless printing belt drawn on round a back-up cylinder and a tension roller, with an impression cylinder, with an engraved roller transferring the ink to the printing belt, and with an ink drier, and a paper web running through all the printing units and guided via an idler roller or idler rollers and a draw roller or draw rollers between the printing belt and the impression cylinder and through the ink drier. The back-up cylinder and the tension roller are arranged movably. The invention also relates to a process for lifting off a printing belt in a flexographic printing machine.
In conventional flexographic printing machines, a printing cylinder is used as a plate carrier or a rubber cylinder with a structured surface. The disadvantage of these printing cylinders carrying the negative printing pattern on their surface is that they have to be exchanged for each printing order. U.S. Pat. No. 3,518,940 therefore proposes a printing mechanism comprising an endless belt composed of polyethylene terephthalate, on which the flexible printing plates are mounted.
Messrs. Conprinta Ltd., Zurich, further developed the flexographic printing machines which have an endless printing belt and described them in their prospectus "Flexographic Printing Presses". The basic principle of these flexographic printing machines is illustrated in FIG. 1. A dimensionally stable endless belt 11 is drawn on a back-up cylinder 12 and a tension roller 14. Flexible printing plates 16 are fastened to the outer face of this endless belt, thus forming a printing belt 10. The printing ink is transferred to the printing plates 16 from an engraved roller 18 which dips into an ink bath (not shown). Excess ink is scraped off by a doctor blade 20 designed in the form of a negative angle. The paper web 22 guided in the direction of the arrow is pressed onto the printing plates by an impression cylinder 24 and printed on one side.
A short printing belt 10' with an endless belt 11'comprising only a few printing plates 16' and tensioned by a tension roller 14' is represented by broken lines.
A cut-out from the printing belt 10 is shown in FIG. 2. The endless belt 11 serving as a carrier consists of a polyester film, for example 0.25 mm thick, especially polyethylene terephthalate. The physical properties of the transversely and longitudinally stretched film material are the same in all directions. The uniformity extends over a wide temperature and humidity range. Furthermore, the film-like belt material has a good elongation and impact resistance in the transverse and longitudinal directions. Finally, the flexible film material of the endless belt is chemically resistant and withstands oils, greases, printing inks, etc. The longitudinal sides of the endless belt 11 are provided with a perforation 26, through which engage knobs of the spiked disks transporting the belt in a known way. The printing belt can thereby be prevented from sliding on the rollers.
The flexible printing plates 16 conventionally consist of a photopolymer material or rubber and are glued to the endless belt 11 by means of a suitable adhesive. In the present example, the printing plates 16 have a structured surface 28 which produces the printing pattern.
The advantage of the above-described printing belt 10 is that the flexible printing plates 16 do not have to be mounted on the printing cylinder, but can be glued to the endless belt which is easily removable from the flexographic printing machine after use. All the printing belts used can be rolled onto a tube and stored in a space-saving way.
At present, flexographic prints of a width of up to 2.5 m are produced by means of printing belts of a length of up to 4.5 m. The flexographic printing machines are allowed to run at high speed and are equipped with checking instruments for the automatic monitoring and recording of the paper webs.
Flexographic printing machines according to the Conprinta system are conventionally equipped with 3 to 10 printing units. Any printing unit not participating in the current printing process can be equipped for the next working step during that available idle time. This usually takes 5 to 15 minutes. During this, the printing belt no longer required is removed, the new printing belt is fitted and the printed ink is exchanged.
Of course, instead of the printing belt, individual printing units can also contain a conventional structured or non-structured printing cylinder. This applies mainly to smooth or structured lacquering or to the application of a base color.
Flexographic printing machines of the type mentioned in the introduction work with back-up cylinders and tension rollers which have bearings fastened rigidly to the machine frame. After the paper web is positioned correctly (the color patterns printed on one another must match exactly), the impression cylinder is pressed onto the endless printing belt drawn over the back-up cylinder, the paper web lying between the printing belt and the impression cylinder. Likewise, the engraved roller which applies the ink is laid against the printing belt in the region of the back-up cylinder.
During normal printing, it occasionally happens that a printing belt has to be taken out of operation for a short time, for example if ink dries on one or more printing plates. For this purpose, the corresponding impression cylinder is raised, as a result of which the paper web also lifts off from the printing belt. The printing belt running at the same speed as the paper web can now be stopped and the appropriate printing plate or printing plates cleaned. After the printing belt has accelerated to normal speed once more, the impression cylinder together with the paper web is pressed against the printing belt again.
The paper misprinted during this operation is detected by an automatic monitoring system and separated out later.
The disadvantage of uncoupling and lifting off the impression cylinder is that the tension of the paper web is changed or even cancelled. This can result in changes in position of the paper web in all the printing units. The entire installation often has to be re-adjusted in such cases.
Attempts have been made, by fitting a compensating linkage, to ensure a compensation of tension in the paper web when the impression cylinder is lifted off. However, such compensating linkages involve a high outlay in every respect.
British Application No. 2,102,733 makes known a printing machine with four printing units which are arranged round a common impression cylinder. A web of material to be printed is guided round the impression cylinder. Each of the printing units serves for applying a special ink and comprises a flexible printing belt which is driven as a result of frictional force and which can be laid individually against a common impression cylinder and removed from it. The back-up cylinder pressing the printing belt onto the impression cylinder or onto the web of material guided on it is arranged on the first carrier, and the engraved roller coating the ink onto the printing belt is arranged on the second carrier. When there is an interruption in printing, for example, in order to lift off the back-up cylinder by means of the first carrier the engraved roller has to be lifted off beforehand by means of the second carrier. There must always be two separate cylces of movement, with the inclusion of the engraved roller, this involving a high outlay and being complicated.
The inventor made it his object to provide a flexographic printing machine of the type mentioned in the introduction, in which a printing belt can be taken out of operation, without engraved roller being displaced or the tension of the paper web being impaired, but the prime costs and operating costs of the flexographic printing machine are to be no higher or not appreciably higher than hitherto. A process for lifting off a printing belt in a flexographic printing machine of the above-mentioned design is also to be developed.